1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for horn antennas, and more particularly to horn antennas which can operate in multiple frequency bands.
2. Description of the Related Art
Conventional electromagnetic waveguides and horn antennas are well known in the art. A waveguide is a transmission line structure that is commonly used for microwave signals. A waveguide typically includes a material medium that confines and guides a propagating electromagnetic wave. In the microwave regime, a waveguide normally consists of a hollow metallic conductor, usually rectangular, elliptical, or circular in cross section. This type of waveguide may, under certain conditions, contain a solid, liquid, liquid crystal or gaseous dielectric material.
In a waveguide, a “mode” is one of the various possible patterns of propagating or standing electromagnetic fields. Each mode is characterized by frequency, polarization, electric field strength, and magnetic field strength. The electromagnetic field pattern of a mode depends on the frequency, refractive indices or dielectric constants and relative permeabilities, and waveguide or cavity geometry. With low enough frequencies for a given structure, no transverse electric or transverse magnetic mode will be supported. At higher frequencies, higher modes are supported and will tend to limit the operational bandwidth of a waveguide. Each waveguide configuration can form different transverse electric and transverse magnetic modes of operation. The most useful mode of propagation is called the Dominant Mode. Other modes with different field configurations can occur unintentionally or can be caused deliberately.
In operation, a waveguide will have field components in the x, y, and z directions. A rectangular waveguide will typically have waveguide dimensions of width, height and length represented by a, b, and l respectively. The cutoff frequency or cutoff wavelength (for transverse electric (TE) modes) can be represented as:                     (                  f          c                )            mn        =                  1                  2          ⁢          π          ⁢                      μɛ                              ⁢                                                  (                                                m                  ⁢                                                                           ⁢                  π                                a                            )                        2                    +                                    (                                                n                  ⁢                                                                           ⁢                  π                                b                            )                        2                                                  and        ⁢                                  (                  λ          c                )            mn        =          2                                                  (                              m                a                            )                        2                    +                                    (                              n                b                            )                        2                              where a is the width of the wider side of the waveguide, and b is a width of the waveguide measured along the narrow side, c is the speed of light, ε and μ are the permittivity and permeability of the dielectric inside the waveguide, and m, n are mode numbers. The lowest frequency mode in a waveguide is the TE10 mode. In this mode, the equation for the signal wavelength at the cutoff frequency reduces to λc=2a. Since waveguides are generally designed to have a static geometry, the operational frequency and bandwidth of conventional waveguides is limited.
Horn antennas are essentially open-ended waveguides in which the walls are gradually flared outwardly toward the radiating aperture. Horn antennas can be designed to support a particular mode, depending on the desired antenna radiation pattern. Generally, horn antennas operate at a specific frequency or within a frequency band.
To overcome the frequency and bandwidth limitations, International Patent Application No. PCT/GB92/01173 assigned to Loughborough University of Technology (Loughborough) proposes that a frequency selective surface (FSS) can be used within a waveguide to influence the frequency response. An FSS is typically provided in one of two arrangements. In a first arrangement, two or more layers of conductive elements are separated by a dielectric substrate. The elements are selected to resonate at a particular frequency at which the FSS will become reflective. The distance between the element layers is selected to create a bandpass condition at a fundamental frequency at which the FSS becomes transparent and passes a signal. The FSS also can pass harmonics of the fundamental frequency. For example, if the fundamental frequency is 10 GHz, the FSS can pass 20 GHz, 30 GHz, 40 GHz, and so on. Of course, if one of the harmonic frequencies happens to coincide with the resonant frequency of the elements, for example if the elements are selected to resonate at 30 GHz, the FSS will be reflective and not pass that particular frequency.
Alternatively, FSS elements can be apertures in a conductive surface. The dimensions of the apertures can be selected so that the apertures resonate at a particular frequency. In this arrangement, the FSS elements pass signals propagating at the resonant frequency. Any other electromagnetic waves incident on the FSS surface are reflected from the surface.
In a multi-band waveguide or horn antenna, the FSS can form a second horn within a first horn wherein the second horn and first horn are tuned to different frequencies. This concept is not without its drawbacks, however. In particular, the horn proposed by Loughborough can generate grating lobes, which is electromagnetic energy that is scattered to uncontrolled directions. Grating lobes result from transmitted and scattered plane waves which do not obey Snell's laws of reflection and refraction. Causes of grating lobes are relatively large inter-element spacing within the FSS, large angles of incidence of plane wave with respect to surface, and/or both. Importantly, grating lobes adversely effect horn antenna performance and should be avoided. Accordingly, there exists a need for waveguides and horn antennas which can incorporate FSS's for multi-band operation, yet which can operate without generating grating lobes.